This invention relates generally to position measurement for numerically controlled machines. In particular, this invention relates to the measurement of the absolute position of a movable machine member relative to a fixed reference location.
It is desirable for numerical control of movement of machine members to produce position measurement signals which are unique for all locations within a range of movement. This type of measurement, called herein absolute position measurement, has the advantage of providing precise information as to the location of a movable member within its range irrespective of whether the location has been recorded and maintained during a period when the member is not subject to numerical control, such as during maintenance of the numerical control device.
Absolute position measurement for movable members of numerically controlled machines is limited by the opposing requirements of resolution and axis range. Absolute position measuring systems are known for full machine range using plural measuring devices, each measuring device operating at a different resolution and each serving to provide unique measurement over a portion of the machine axis range. When such systems are implemented using measuring devices incorporating a rotating element, substantial gear reductions are required from the machine member drive to the measuring device used to produce the coarsest measurement resolution. When such systems are implemented using linear measuring devices the number of scales required to produce unique measurement signals over the full range of the machine axis becomes excessive.
A further disadvantage of the known absolute measuring systems, is their susceptibility to errors attributable to the measuring devices themselves or to the mechanical drive mechanisms associated therewith. The errors arising from these sources have an adverse affect on the overall machine positioning accuracy.
It is, therefore, one object of the present invention to provide an absolute position measuring apparatus and method operative over the full range of motion of a movable machine member and requiring no more than two measuring devices, each measuring device producing a unique measurement signal which is repeated over the range of the machine member motion.
It is a further object of the present invention to provide an absolute position measuring apparatus and method using two cyclic measuring devices and which is tolerant of errors of the relative position measurement made by the two measuring devices.
It is a still further object of the present invention to provide an absolute position measuring apparatus and method operative over the full range of motion of a movable machine member using a master measuring device and a vernier measuring device, each measuring device producing a cyclic measurement signal as the machine member moves over its range, the vernier cyclic measurement signal being repeated no more than once more within the range of motion than the master cyclic measurment signal.
It is a still further object of the present invention to provide an absolute position measuring apparatus operative over the full range of motion of a movable machine member having two measuring devices each having a rotatable element and each producing cyclic measurement signals, the rotating elements being driven relative to one another so that one measuring device produces no more than one more cycle of its measurement signal than the other measuring device over the range of axis motion to be measured.
Further objects and advantages of the present invention shall be made apparent in the attached drawings and the associated description thereof.